[ASAHI KASEI]
Measurement of Battery Voltage
The voltage that is divided by the appropriate registers is input to BTI_1 pin or BTI_2 pin in order to measure battery voltage.
TPVREFIN is referred to as full scale.
A/T I/F is used for the selection of input channels, BTI_1 or BTI_2. The input voltage to BTI_1 and BTI_2 must NOT exceed VA
voltage.
If the impedance of input signal is high, enough tracking time is required to charge the internal capacitors through BTI_1 or BIT_2.
And if the input impedance of TPVREFIN is high, clock frequency of TADCK must be low.
<The relationship between BIT_1/2 input impedance and tracking time>
For example, if the external battery voltage that is divided by two 30k Ω resistors is input to BTI_1 or BTI_2, the input impedance is 15
k Ω. In this case, tracking times should be at least 50µs. If the input impedance is 30 k Ω, tracking time should be at least 100 µs.
<The relationship between TPVREFIN and TADCK>
For example, external reference voltage that is divided by 10 k Ω and 30k Ω is input to TPVREFIN, input impedance of TPVREFIN is
about 7.5k Ω. In this case, the clock frequency of TADCK should be 160kHz or less.
As full-scale error increases with high input impedance, high input impedance is not suggested. If the full scale error and long
conversion time can be ignored, high input impedance may be used.
As the measurement of battery voltage and detecting pen interrupt can be controlled independently, micro-controller can knows the
interrupt while the battery measurement.
TPVREFIN
battery 1
battery 2
BT1
12bit
ADC
BT2
Figure 21. Block Diagram of Battery Measurement
<Revision 0.9a>
34
July 00
AKM [ ASAHI KASEI MICROSYSTEMS ]
